FIG. 1 illustrates a prior art cryogenic air separation system 10 which produces gaseous oxygen, gaseous nitrogen and liquid oxygen. In system 10, a stream S4 of high pressure feed air passes through a primary heat exchanger 12 where it is cooled by indirect heat exchange against return streams S3 of product gaseous oxygen, S2 of waste nitrogen and S1 of product nitrogen. A second, lower pressure feed air stream S5 also passes through and is cooled by indirect heat exchange in primary heat exchanger 12. Air stream S5 is turbo expanded and is fed to a high pressure column 14. High pressure air stream S4, after exiting from primary heat exchanger 12, passes through an oxygen boiler 16, where heat exchange occurs as a result of flow of a liquid oxygen stream 18 from pump 20.
Within column 14, the air feeds undergo a preliminary separation into liquid fractions of crude oxygen and substantially pure nitrogen. The nitrogen outflow passes via pipe 22 through a condenser/reboiler 24. A portion of the resultant nitrogen-rich liquid stream S7 then is passed through subcooler 26 and is fed as a liquid reflux to the top of low pressure column 28.
The crude oxygen fraction from lower column 14 is fed via pipe 30 as stream S6 through subcooler 26 and into a side feed of upper column 28. Within upper column 28, the fluids are separated by cryogenic distillation into a nitrogen-rich vapor and an oxygen-rich liquid. The nitrogen-rich vapor is withdrawn from upper column 28 via pipe 32, is passed through subcooler 26 and primary heat exchanger 12, to exit as product nitrogen gas stream S1. A waste nitrogen stream emerges from upper column 28 via pipe 34 and proceeds through the same route to waste nitrogen stream output S2. The oxygen-rich liquid is removed from the bottom of low-pressure column 24 via pipe 36. Another portion of oxygen-rich liquid is removed via pipe 37, pumped to a higher pressure by pump 20 and is then vaporized and warmed to obtain product oxygen stream S3. Further details of the operation air separation system 10 can be found in U.S. Pat. No. 5,108,476 to Dray et al.
As can be seen from an examination of air separation system 10, the cryogenic distillation of air is a process which requires extensive heat integration and exchange between various inlet and outlet streams. The heat exchange processes can be classified as follows:
1) Primary heat exchange (occurring in primary heat exchanger 12) wherein the incoming feed air is cooled down to the process temperatures against product and waste streams; PA1 2) Product oxygen boiling (in oxygen boiler 16) where liquid oxygen product is boiled against a condensing air stream S4; PA1 3) Subcooling of liquid streams (occurring in subcooler 26) where cold product and waste streams are warmed against various liquid streams; and PA1 4) Reboiling/condensing (in unit 24) where a nitrogen-rich vapor condenses against an oxygen-rich liquid.
The provision of individual, separate heat exchange units to provide the necessary heat transfer functions constitutes a substantial expense item in air separation system 10. The art is replete with various designs for improved heat exchange units. One such design is shown in U.S. Pat. No. 5,438,836 to Srinivasan et al., which discloses a downflow, reboiling/condensing, plate and fin heat exchanger wherein nitrogen-rich vapor condenses against an oxygen-rich liquid stream.
Agrawal et al., in U.S. Pat. No. 5,275,004, suggest performing a reboiling/condensing heat exchange in the primary heat exchanger and that the primary heat exchanger can also include the subcooling heat exchange function. The heat exchange unit described by Agrawal et al. employs a counterflow configuration, wherein streams to be heat exchanged flow in opposite directions through the exchange structure.
A significant problem with the heat exchange structure taught by Agrawal et al. is that its implementation in a size suitable for incorporation into an operating cryogenic separation plant leads to an exchanger with total core lengths that are costly and difficult to manufacture.
Accordingly, it is an object of this invention to provide a heat exchange unit for an air separation system wherein the number of heat exchange cores is reduced.
It is another object of this invention to reduce waste and product stream pressure drops as a result of the use of shorter heat exchange sections, including subcooling sections.
It is a further object of this invention to reduce the number of headers required for coupling of feed streams to the heat exchange system.